Shorthand for Third Culture Kid; someone who has spent a good chunk of their developmental years in cultures other than their national or ethnic groups. This can result in cultural identity confusion, multiple language skills, a high degree of maturity and independence, and a discomfort with settling in any one place for too long.

Guy: Hey, Beth's pretty comfortable with black people despite being a white girl.
Girl: She's a TCK -she grew up in South Africa and was surrounded by blacks.